The study was initiated to characterize the virus-host cell relationship of isolates of endogenous viruses of the Southeast Asian mouse and the Chinese hamster. Initial findings concerning the association of viral precursor particles of type A with the mitotic apparatus, the microtubule system and specific chromosome sites, the kinetochores, led us to investigate possible loci in the area of the primary constriction of individual chromosomes that may determine the assembly of viral precursor particles. A spreading technique for whole mount electron microscopy of isolated chromosomes was developed which allowed clear demonstration that the intracytoplasmic type A particles are associated with kinetochore plates of all chromosomes. The presence of specific chromosomes with regulatory gene loci near the kinetochores determining viral maturation can be excluded.